MARYLAND GEOLOGICAL SURVEY 85 



The Proterozoic forms of calcareous algae have been described under 

 six genera, but all of the Cambrian and Early Ordovician forms have been 

 referred to the single genus Cryptozoon. The basal Conococheague 

 species, consisting of a wide, flat basal portion of laminae growing into 

 numerous head-like masses large at the center and small along the edges, 

 was described long ago by Hall as Cryptozoon proliferum. The second 

 species, with laminae of equal undulations, is described in this volume 

 as new. 



These two types of structure are often associated together in such 

 numbers that they form a true reef. Sometimes only one of the species 

 will be represented in the reef, though occurring in such great numbers 

 as to completely fill the rock. A reef composed entirely of Cryptozoon 

 proliferum is well exposed in a cut along the Norfolk and Western Bail- 

 road about one mile southwest of Antietam Station, Maryland (see 

 pi. IX, fig. 1), where the highly tilted limestones expose the individual 

 colonies of the alga to good advantage. Similar reefs of C. proliferum 

 were observed along the northern line of outcrop from the state line south- 

 east to Chewsville. The base of the line of low hills about a mile west of 

 Ringgold gives numerous specimens of this species. The outcrops of the 

 basal beds along the line five miles southeast of Hagerstown show reefs 

 of the new species Cryptozoon undulatum most commonly. 



These reefs of calcareous algae are of interest and practical value from 

 the standpoint of structural geology because they afford an exact criterion 

 for determining the top or bottom of a stratum. In areas of highly folded 

 strata such as the Appalachian Valley, this determination is frequently 

 highly important and sure methods are few. The broad upfolds of the 

 laminations and the narrow sharp down folds register the upper and 

 lower sides respectively of the stratum without a doubt. 



Still a third type of strongly laminated Cryptozoon structure occurs 

 near the top of the Conococheague limestone in both the eastern and 

 western areas of outcrop in Maryland. No specimens have been obtained 

 free from the matrix, but natural sections in the rock show that the 

 undulations are 18 or more inches in width and that the zone of strong 



